Multilateration system and method

ABSTRACT

A multilateration system and method includes a plurality of receiver stations for receiving signals from an aircraft, and a controller that derives the position of the aircraft by applying a multilateration process to outputs from the receiver stations. For this purpose, the controller determines the altitude of the aircraft and selects a multilateration process that is to be used for position determination, based on the determined altitude.

This invention relates to a multilateration system for aircraftlocation.

BACKGROUND OF THE INVENTION

Many types of aircraft transmit coded signals for example SecondarySurveillance RADAR (SSR) codes such as a so-called mode A, C or S codeswhich may be used by ground based receivers to determine the aircraft'sposition. The position is determined from noting the time of arrival atthe receivers and by using this with knowledge of the positions of thereceivers themselves. GB2250154A and GB 2349531A disclose suchmultilateration systems. These systems utilise four receiver stationscontrolled from one master station in order to establish the aircraft'sposition in three dimensions.

SUMMARY OF THE INVENTION

The present invention arose from a consideration of situations when onereceiver station fails to receive the transmitted code, the code isgarbled or when one receiver station develops a fault. Consideration wasalso given to situation where aircraft are unable to transmit mode Acodes.

According to the invention there is provided a multilateration systemcomprising a plurality of receiver stations for receiving signals fromaircraft and a controller to apply a multilateration process to'outputsof the receiver stations, indicating receipt of the signal, to derive aposition of the aircraft characterised in that the controller determinesthe number of active receiver stations receiving the code, determinesthe type of code and performs a multilateration process in accordancewith the determination to provide a position of the aircraft.

In certain situations it will be appreciated that there may beinsufficient receiver stations receiving the transmitted code todetermine the location with a great deal of accuracy. For example, threereceiver stations will be able to provide a two dimensional positionwhich may in some circumstances be useful.

Some aircraft are equipped with mode A SSR transponder but are able totransmit other codes for example mode C. Mode A codes include a uniqueaircraft identifier and thus can distinguish a mode A code transmittedby one aircraft from second mode A code transmitted by another. In somecases the mode A transmission may be corrupted and hence not usable.Other codes may not include such a unique identifier. Preferably, insuch a case the multilateration process will include a reference totracking system to distinguish between possible sources. In the trackingsystem, a table is produced on the basis of the returned signals whichis revised over time.

It will be appreciated that a multilateration process involvessignificant computational resources and it will be advantageous in someapplications to perform the different multilateration processesavailable according to the accuracy required. Preferably, this isselected on the basis of the source aircraft's altitude. This has beenfound to be advantageous since the uncertainty in position in terms ofground position of the aircraft will increase with an increase inaltitude. Hence, when the aircraft is at a high altitude full threedimensional multilateration will be required whereas at a relatively lowaltitude two dimensional multilateration will suffice. In the describedembodiment, for heights between high and the low altitude thresholds atwo dimensional multilateration is performed which is augmented with thealtitude of the aircraft.

The transmitted code may include data concerning the altitude of theaircraft. This may be determined by the aircraft itself or by groundbased means. In the case of Secondary Surveillance Radar (SSR) codes, amode C code includes altitude information. In this case, the controllermay perform a two dimensional multilateration process using some of thereceivers and using the value of the altitude to arrive at a threedimensional location.

The invention also provides a multilateration method.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows in schematic form a multilateration system 1 operating inaccordance with the invention;

FIG. 2 is a block diagram of a controller for the multilateration systemaccording to the invention;

FIG. 3 shows a track table generated and stored by the controller ofFIG. 2;

FIG. 4 is a diagram that shows altitude and range correlation error;

FIG. 5 is a graphic illustration of relationship between aircraftaltitude and the selection of a multilateration process;

FIG. 6 illustrates vector consideration by the controller according tothe invention; and

FIG. 7 a-7 d show alternatives for decision logic according to theinvention.

DETAILED DESCRIPTION OF THE DRAWINGS

As is shown in FIG. 1, a multilateration system 1 includes a pluralityof receiver stations 2 to 6 positioned at a number of locations on theground. These receive a transmitted signal include a code from atransponder mounted on an aircraft 7. The code is a SecondarySurveillance RADAR code which may be a mode A, mode S, mode C or may bean unknown mode of code.

Receiver station 4 is termed a master station because it includes acontroller 20 which uses the data from the receivers to perform themultilateration process. (In alternative embodiments it need not beco-located with the receiver.) The data from the receiver stations 2, 3,5 and 6 is passed to the master station 4 over data links 8. Thecontroller 20 is microprocessor based and is shown in greater detail inFIG. 2. It includes a number of input ports 21 to 24 linked to the datalinks 8 and hence to the receiver stations. The input ports areconnected to a correlator 25 which forms the data into sets whichoriginate for particular transmissions of codes or events. The eventsare correlated by reference to time. Thus, if a mode A, mode C, mode Sor an unknown mode code arrive within a certain time frame then they areconsidered to originate from the same aircraft. The correlator also tiesup Time Of Arrival information from all receivers for a giventransmission as a so-called TOA Vector which may be of arbitrary lengthdependent upon the receivers that received a particular emission of acode. The vectors are then stored in memory 26 as tables of times ofarrival and associated codes.

The vector held in the memory 26 is then accessed by a locator 27 thatincludes decision logic which analyses the data to determine for eachvector a number of criteria, as will be described later, and then toselect the appropriate multilateration process to be applied to thedata. The vector together with an instruction as to the process to beapplied is then passed to a multilateration processor 28 and thepertinent multilateration process applied. The position is then used topopulate an entry in a track table 29. This is shown in FIG. 3. Each rowof the table is a termed a “track” and includes the codes whether modeA, C or S Airframe Address, and a position expressed as co-ordinates x,y and z. The track table is accessible to a plot association block 30.This is able to associate different responses from the aircraft to forma single track entry in the track table 30 from multiple track entries.

The plot association block 30 provides an output to a formatter 31 whichplaces the tracks into the correct format for input into a trackersystem 32. The tracker system 32 provides an output to an air trafficcontrol system 33 for displaying the tracks to a human air trafficcontrol officer.

The selection criterion referred to above include the following:

1. Mode type, whether the received signal is mode A, S or C or anunknown mode.

2. Number of receiver stations providing data to the data set.

3. Whether the data indicates altitude or altitude is available fromanother system or altitude may be assumed.

4. Desired accuracy for the positioning

The altitude may be determined in a number of ways. If the received codeis mode C then this includes an altitude provided by the aircraft itselfby use of an onboard altimeter for example. (In some embodiments,altitude may be provided from an earlier multilateration on the sameaircraft or from knowledge of the aircrafts flight path which mayrequire the use of a particular altitude for example.)

As is shown in explanatory FIG. 4, the aircraft's altitude has anuncertainty and this will translate to an uncertainty in thecorresponding ground position. Arising from an appreciation of this, theinventors have determined that a satisfactory multilateration at lowlevels may be achieved by assuming that the aircraft is at zero altitudeand co-planar with the receivers and hence a two dimensionalmultilateration process may suffice. For intermediate levels, a twodimensional multilateration augmented with altitude information may beused and a three dimensional multilateration using four or morereceivers will be required at high altitudes. This banding is shownschematically in FIG. 5.

The decision logic in locator 27 (FIG. 2) considers each vector in themanner illustrated in FIG. 6. The TOA vector is accessed and, indecision step 60, the vector length is considered. If the vector lengthis four returns or more then branch 61 is followed to the next step 62.

In step 62, the vector is considered and the receivers making thereturns determined. It will be appreciated that even though four returnsare available in a practical system these may not be ideally spread.Hence, if the geometry of the group of receivers providing the returnsin the vector are not such as to give sufficient accuracy then anegative branch 63 is followed to step 64. If the geometry does offersufficient accuracy then branch 65 is followed to step 66. In step 66 afull three dimensional multilateration is instructed.

Returning to step 60, if the vector length is less than four thennegative branch 67 is followed to step 64. In step 64, a decision ismade as to whether or not a two dimensional multilateration or anaugmented two dimensional multilateration is to be performed using thebarometric height indicated in a mode C emission. The step is dividedinto various choices dependent upon both the SSR code associated withthe data input and the certainty of which type the code might be. Thefollowing cases exist:

-   -   1. Mode S    -   2. Mode A    -   3. Mode C    -   4. Either Mode A or C (i.e. not known which)

Note that military variants have been ignored for clarity.

Each of these cases has its own decision logic. The logic used isdependent upon the extent of information in the Track Table. The processis illustrated for three cases as shown in FIGS. 7 a to 7 d.

In FIG. 7 a, the first case process is represented for an examination ofthe received code in which the code is a mode S code. In a firstconsideration step 70, the received code has an airframe address whichis compared with the tracks in the track table 29 to see if the airframeaddress is already present as an entry. If it is, then the processfollows branch 71 to the next consideration step 72 in which the tracktable is examined for the availability of the mode C altitude for theparticular track. If the mode C altitude is present, then the branch 73is followed and the next consideration step 74 made. In step 74,consideration is made as to whether or not the altitude is contained inthe current mode S received code. If it is, then branch 75 is followedand the track is used in process 76 to perform a two dimensionalmultilateration using the altitude in the mode S code and the track inthe track table is updated with the position in terms of x,y and zco-ordinates.

If in the step 74, the altitude is not contained in the current mode Sthen the negative branch 77 is followed to process 78. In this process amultilateration is performed with the altitude from the track tableavailable from the last mode C return. The result is then passed to amatching process 79 which compares the result with one extrapolated forthe track. If there is a match within a certain threshold, then thetrack in the track table is updated with the new position. If there isno match, then the negative branch 80 is followed to process 81. (Inessence this will be because the received code is a new aircraftentering the air traffic control area.) The step 81 results in amultilateration process being performed at zero feet and the track tableis updated to include a new track bearing a flag indicating that it isnot to be output from the system as it has insufficient positionalaccuracy. This track will be updated as more codes are received and whenthe accuracy is acceptable the flag will be brought down permitting thetrack to be output from the system.

Returning back to step 70, if the airframe address is not present in thetrack table then negative branch 82 is followed to the step 74. If thealtitude is contained in the current mode S code, then the positivebranch 83 is followed to step 76. If the attitude is not contained thena negative branch 84 is followed to process 81.

Returning to step 72 if the result of the consideration of the mode Caltitude being in the track table is negative then branch 85 is followedto step 74.

In step 78, the multilateration process is performed using the altitudein the track table. However in step 76 the multilateration process willbe carried out on the basis of that in the current mode S code. Themultilateration process to be applied whether 2d or 2d augmented is donewith a consideration of the bandings of FIG. 5. If the altitude isbetween Hmin and H3d then a 2d assisted multilateration process isfollowed and a flag is added indicating reduced accuracy. If thealtitude is below hmin then the multilateration process is a 2d processand the result is marked by a flag as full accuracy. If the altitude isabove h3d then the locator does not provide an output and the tracktable is not updated.

In case 2 shown in FIG. 7 b the currently received code is a mode Acode. This code is compared in step 90 with each track in the tracktable. The matches are then considered in turn in step 91 as to whetheror not the track includes a Mode C altitude. If it does then branch 92to process 93 is followed or if not branch 94 to process 95. If thereare no matches then branch 96 is followed to process 97.

In process 93, the altitude is compared with the banding as before toselect the multilateration process to be applied. If the altitude isbetween hmin and h3d then a 2d assisted multilateration process isapplied using the altitude from the track table for the matching trackflagging the result as reduced accuracy. If the altitude from the tracktable is below hmin then a 2d multilateration is performed marking theresults as full accuracy. if the altitude is above h3d then there is nooutput from the locator.

In process 95 a 2d multilateration is carried out and the result flaggedas not to be output from the system. The results for position fromprocess 93 and 95 are passed to a comparison step 96. In this comparisonstep the position in terms of x,y and z co-ordinates is compared with anextrapolated position for the track. If there is a match the results areused to update the track in the track table if there is no match thenthe loop branch to the step 90 is followed. (Matching may be done interms of x,y and z or x,y and a z determined from a mode C transmissionsin some embodiments.)

In the case of no matches in the code or on the x, y, z co-ordinates inprocess 96, a 2d multilateration process is carried out and the resultsmarked as not to be output from the system.

In the case of the received code being a mode C code the steps are shownin FIG. 7 c. In a first step 100, the matches for the received code inthe track table are identified. Then each match has a multilaterationprocess 101 applied to it using the altitude from the track entry in thetrack table. If the altitude is between hmin and h3d then a 2D assistedmultilateration is carried out with the results mark as reducedaccuracy. If the altitude is below hmin then a 2d multilaterationprocess is carried out and marked as full accuracy. If the altitude isover h3d then no output results from the locator. The resultantmultilateration position is compared with process 102 to an extrapolatedposition for the particular track form the track table. If there is amatch then the track is updated. If there is not a match, then thenegative branch is followed back to the step 100.

If there are no matches for the code or co-ordinates then process step104 is carried out for the received mode C code as it is being receivedfrom an aircraft entering the monitored airspace. In process 104 a 2d or2d assisted multilateration process is carried out on the basis of thealtitude in the received mode C code. In this process if the altitude isbetween hmin and h3d a 2d assisted multilateration process is carriedout with the result marked as reduced accuracy. If the altitude is belowhmin then a 2d multilateration process is carried out and the resultsmarked as full accuracy. If the height is above h3d then there is nooutput from the locator.

The 3D multilateration process will involve four or more of thereceivers as disclosed in GB225014 or GB 239531 for example. However, ifthe altitude is known, only two time difference of arrival figures needbe determined.

The foregoing disclosure has been set forth merely to illustrate theinvention and is not intended to be limiting. Since modifications of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and equivalents thereof.

The invention claimed is:
 1. A multilateration system comprising: aplurality of receiver stations for receiving a signal from an aircraft;and a controller that derives the position of the aircraft by applying amultilateration process to outputs of the receiver stations, whichoutputs indicate receipt of said signal, wherein the controllerdetermines i) determines an altitude of the aircraft; ii) chooses amultilateration process based on the determined altitude; and iii)performs the chosen multilateration process to provide a position of theaircraft.
 2. The system as claimed in claim 1, wherein the controllerperforms the multilateration process using said outputs of the stationsand the altitude of the aircraft.
 3. The system as claimed in claim 1,wherein: the controller uses said outputs to create a track for theaircraft; and said track is stored in a memory.
 4. The system as claimedin claim 3, wherein the track comprises at least one of a mode typeindicator, a mode code, and a position.
 5. Currently Amended) The systemas claimed in claim 3, wherein: the controller comprises a locatorhaving logic for comparing i) a currently determined position providedby the chosen multilateration process with ii) a predicted positionextrapolated from a track; and if comparison is within a certainthreshold, the logic concludes that there is a match and updates thetrack with the currently determined position.
 6. The system as claimedin claim 1, wherein: the controller comprises a locator having controllogic for i) determining the number of receiver stations providingusable outputs, ii) determining from the outputs the geometry of thereceiver stations making the outputs and whether the geometry willsupport a three dimensional multilateration, and iii) carrying out sucha multilateration; and in the absence of such a determination thecontrol logic carries out a two dimensional or two dimensional augmentedmultilateration.
 7. An air-traffic control system for monitoringair-traffic, said air-traffic control system comprising: amultilateration system that includes a plurality of receiver stationsfor receiving a signal from an aircraft; and a controller that derivesthe position of the aircraft by applying a multilateration process tooutputs of the receiver stations, which outputs indicate receipt of saidsignal, wherein the controller i) determines an altitude of theaircraft; ii) chooses a multilateration process based on the determinedaltitude; and iii) performs a chosen multilateration process to providea position of the aircraft; means for displaying at least some of trackinformation provided by the chosen multilateration system to a systemoperator.
 8. A multilateration method of determining the position of anaircraft emitting a code, said method comprising: receiving the code ata plurality of receiver stations; and deriving a position of theaircraft by applying a multilateration process to outputs of thereceiver stations; wherein said step of deriving a position of theaircraft comprises, determining an altitude of the aircraft; choosing amultilateration process based on the determined altitude; and performingthe chosen multilateration process to provide the position of theaircraft.
 9. The method as claimed in claim 8, wherein the altitude is abarometric altitude obtained from a received code.
 10. A method asclaimed in claim 8, wherein said step of choosing a multilaterationprocess comprises comparing the determined altitude with thresholdvalues to determine the multilateration process.
 11. A method as claimedin claim 8, wherein data contained in said outputs of said receiverstations are stored as tracks.
 12. The method as claimed in claim 11,further comprising: comparing a current position provided by the chosenmultilateration process with extrapolated positions from the tracks todetermine a match; and in the event of a match, updating the track withthe current position.
 13. The air-traffic control method comprisingdetermining aircraft positions using a method as claimed in claim 8, anddisplaying the aircraft positions to an operator.